1. Field of the Invention
The present invention relates to the fabricating of an integrated circuit. More particularly, the invention provides an ArF photoresist resin and a preparation method therefor and, more particularly, to a photoresist copolymer suitable for submicrolithography using deep ultra violet (DUV) light as a light source and a method for preparing such a copolymer. Also, the present invention provides a photoresist composition (“photoresist”) including such a resin.
2. Description of the Prior Art
Recently, chemical amplification photoresists have been prevailing in semiconductor devices since they have been found to be highly sensitive to DUV light, which is recognized as a light source suitable for accomplishing the high integration of semiconductor devices. A chemical amplification photoresist generally has a photoacid generator and a matrix polymer having a chemical structure which sensitively reacts with acid.
As for the reaction mechanism of such a photoresist, when the photoresist is exposed through a mask to a DUV light source, protons are generated by the action of the photoacid generator, which then reacts with the main or side chain of the matrix polymer. This reaction increases the solubility of the copolymer in a developing solution by converting the structure of the copolymer, e.g., by decomposing it, cross-linking it or changing its polarity. Therefore, when treated with the developing solution, the copolymer is dissolved at exposed regions and remains undissolved at un-exposed regions, thereby leaving the shape of the mask as a positive image on a substrate.
Meanwhile the resolution of the patterns formed by photolithography is generally proportional to the wavelength of the light source. Thus, finer patterns can be formed as the wavelength is shorter. As a result of the effort to find new light sources suitable to improve the resolution, deep UV (DUV) light was developed for the integration of semiconductor devices into 1 Giga or higher scale.
Generally, photoresists are required to be of high etch resistance and thermal resistance. In addition, the photoresist to be used for an ArF (193 nm wavelength) light source should be developed in a 2.38% tetramethylammonium hydroxide (TMAH) solution. However, in fact, it is difficult to obtain a photoresist resin which satisfies those properties entirely.
For example, resins having a backbone of poly(methylmethacrylate), which is transparent to light of the above short wavelengths, are easy to synthesize. But there are problems in practical application owing to their poor etch resistance and development in TMAH solution. Etch resistance can be improved by introducing aliphatic ring monomers into the main chain. But it is virtually impossible to synthesize a resin having a main chain consisting of aliphatic rings.
In order to solve the problems, people such as those at AT&T (or Bell Laboratory) has developed a resin having a main chain which comprises norbornene, acrylate and maleic anhydride monomers, as represented by the following formula I: 
In Formula I, the maleic anhydride (part A) is used to polymerize aliphatic cyclo-olefin groups, but is dissolved in a 2.38% TMAH solution even in the state of unexposure. This dissolution can be inhibited by increasing the proportion of the y part of Formula I, the t-butyl substituent, in the main chain. If this is done, the z part, functioning to increase the adhesiveness to a substrate becomes relatively small in proportion, which leads to the release of the photoresist from the substrate, e.g. a silicon wafer. As a result, the formation of good patterns is impossible by this method. Bell Laboratory suggested a two-component system including a cholesterol compound as a dissolution inhibitor. This dissolution inhibitor is, however, required to be added in a large quantity, for example, about 30% by weight of the resin, so that Bell Laboratory's resins are in principle problematic for use in a photoresist.